Field of the Invention
The present invention relates to a frequency stabilizing apparatus for an isolated system, which gives a control command to a storage battery capable of adjusting power on an isolated system for power supply.
Description of the Background Art
In a power plant, supply of power from a power generator is adjusted to meet a power demand changed moment by moment. A fluctuation in the power demand described herein includes a fluctuation in power consumed by a customer, and furthermore, a fluctuation in regenerable energy power generation such as wind power generation or solar power generation installed by the customer or a power generation operator, and a fluctuation caused by parallel off due to an accident.
A frequency of a power system has such a feature that the frequency is reduced if consumption is larger than supply and is increased if the consumption is smaller than the supply. In some cases in which a frequency of power is changed greatly from a specified value, however, an apparatus of the customer is affected. It is anticipated that power generating equipment is also influenced if a deviation is further increased, and service interruption occurs in the worst case. In the power plant, therefore, by performing a demand and supply operation, demand and supply balance of the power is taken in such a manner that a frequency of power is maintained to be a specified value. In the case in which a power generator or a power generator controller detects a deviation of a frequency from a reference frequency to be a reference of the frequency, usually, power supplied from the power generator is adjusted depending on the deviation, thereby controlling to return the frequency into the specified value (reference frequency).
In general, the control is delay control. In the case in which a speed of the fluctuation in the demand exceeds an adjusted speed through the power generator, moreover, the control is delayed so that the frequency deviates from the reference frequency for a while. The adjusted speed differs depending on an operating state of the power generator (a total of rated outputs of the power generator which is being operated). For example, if a total value of rated output values of the power generator which is being operated is increased, the adjusted speed is increased and can follow a high change speed.
The power supplied from the power generator has the feature described above. In recent years, introduction of a power generator for converting regenerable energy into electric energy has progressed. The regenerable energy has such a feature that a fluctuation in an output is great and the fluctuation in demand tends to exceed the adjusted speed. In the future from now, therefore, it is supposed that there is a high possibility of the occurrence of an unstable phenomenon in a frequency described above. For this reason, as in the technique disclosed in International Publication No. WO2013/140916 and Japanese Patent No. 4672525, it is variously investigated to add storage battery equipment (hereinafter referred to as a “storage battery”) including a capacitor and an inverter and to stabilize a frequency by causing an output of the storage battery to follow the fluctuation in the demand which cannot be followed properly by the power generator. Such a system stabilizing method is also investigated in a large-scale power system in addition to an isolated island.
Referring to a small-scale isolated system such as an isolated island, influence of demand and supply imbalance (a shift of demand and supply balance) on a frequency is particularly large. As described above, moreover, a shift amount of the demand and supply balance and a frequency deviation (a change in a frequency from the reference frequency) are varied depending on an operating state of the power generator at that time.
On the other hand, in the technique described in the International Publication No. WO2013/140916, a command value to be given to the storage battery is generated based on an interconnection point tidal current value and a power generator change rate limit value. Consequently, a change in the output of the power generator is limited to reduce the frequency deviation, in order not to influence the frequency.
Referring to the technique disclosed in the International Publication No. WO2013/140916, however, also in the case in which the frequency deviation is not perfectly cancelled, the storage battery does not execute control for positively cancelling the frequency deviation after the interconnection point tidal current value is stabilized into a certain value. For this reason, control for stabilizing the frequency is executed by only the control of the frequency through the power generator. As a result, there is a problem in that a slight frequency deviation is present for a comparatively long period of time.